1. Field of the Invention
This invention relates to optically coded documents, i.e., documents comprising an information carrier on which information is recorded in the form of optical markings.
2. Description of the Prior Art
Documents such as identity cards, credit cards, pre-paid payment tokens which can be devalued in stages, travel tickets, entry cards and the like, carrying information in the form of optical markings, are known in many different forms. Most known documents of this type can be counterfeited fairly easily.
A very high degree of protection against forgery can be obtained if the information is fed into the document in the form of diffraction gratings, holograms or similar light-modifying optical markings. For example, a known document which is used as a pre-paid payment token for obtaining goods or services comprises an information carrier made of thermoplastic material with optical markings impressed in it. The markings, representing units of value, have a structure which modifies incident light by diffraction or refraction. When the document is used in a service machine, telephone, etc., the optical markings are tested for genuineness, and a number of markings corresponding to the amount to be paid is cancelled by fusing or perforating them, scraping them off or otherwise obliterating them. The obliteration process requires a relatively large amount of energy, which in many cases is not readily available.
In a document with a plurality of optical markings impressed in thermoplastic material, which modify incident light in a predetermined manner, it has further been proposed to feed in coded information by subsequently obliterating or altering selected optical markings by the action of heat. Here again a relatively large amount of energy is required to obliterate or alter the markings.